Communication systems often transmit data with a clock embedded in a data stream, rather than being sent as a separate signal. When the data stream is received, a clock and data recovery circuit (CDR) recovers the embedded clock and retimes the received data to the recovered clock. Typically, a phase-locked loop (PLL) is used to perform the clock recovery operation. Such a PLL typically includes a phase detector, which receives the input data signal and a clock signal from a voltage-controlled oscillator (VCO). The phase detector generates an error signal, which is a function of the phase difference between the input data signal and the VCO clock signal. The phase detector may also include additional circuitry to generate the reconstructed data.
The data stream is used to transmit digital information at high data rates. For data to be reliably transmitted and received, a system typically has a low bit error rate (BER). One method to estimate the BER is to count the number of data transitions occurring within a certain time frame of a data eye.
The phase detector, oftentimes a linear phase detector, is used to determine an optimal phase sampling point for the incoming data eye. However, such phase detectors rely on the matching of delays between data and clock paths. Accordingly, these phase detectors are notorious for having large phase offsets that change with both process and temperature. Accordingly, a need exists to calibrate out systematic phase offsets of a phase detector.
Furthermore, depending on the type of optical components and fiber lengths within a data communication system, an optimal phase sampling point within a data eye may vary. An example data eye is shown in FIG. 1A, which is a diagram of incoming data. As shown in FIG. 1A, data eye 5 can be asymmetric, leading to a histogram which is also asymmetric. For example, as shown in FIG. 1B, which is a histogram of zero crossings for data eye 5 of FIG. 1A, the number of zero crossings occur asymmetrically over time. While the data is often sampled at a point with minimal zero crossings to reduce the BER of the receiver, the optimal phase sampling point of a data eye varies with different optical components and fiber lengths. Further, this optimal phase sampling point varies from one optical system to another. Accordingly, a need further exists to automatically adjust a phase of a data sampler to an optimal phase location.